I'm so sorry that you are feeling that way- must be horrible.
I have experienced anxiety for the past few years now, it was at its worst near the end of high school when there wasn't an awful lot wrong with my life. These attacks that you are describing resonate greatly with this time, and so does too the frustration in not being able to explain why they were happening.
I would get into these vicious cycles of having an attack, worrying about where the attack came from, and then having another attack due to this.
Something that helped me in these times of heightened anxiety is the acceptance that they were happening for no reason at all- and that it was okay for this to occur.
I know it is hard, and accepting the fact that your just having an attack out of no where is difficult in itself, but at least the worry of where it is coming from is at least sorted.
Overall, sometimes by trying to figure out why your having an attack when the answer is not obvious is only feeding into what your anxiety wants: for you to be fearful of something.
Keep going- I think it is great that you are aware of anxieties presence in the first place, that's a huge step!